1. Field of the Invention
The present invention relates to the field of steam oxidation and more specifically to a method and apparatus for insitu moisture generation in a rapid thermal steam oxidation process.
2. Discussion of Related Art
In the fabrication of modem integrated circuits, such as microprocessors and memories, oxidation processes are used to passivate or oxidize silicon films. A popular method to oxidize silicon surfaces and films such as polysilicon gate electrodes and substrates is to use a steam (H.sub.2 O) oxidation process. In such cases water vapor (H.sub.2 O) is brought into an oxidation chamber to react with the silicon surfaces to form silicon dioxide.
Present steam oxidation processes generally take place in multi-wafer resistively heated "hot wall" furnaces. Present steam oxidation processes typically use a pyrogenic torch or bubbler located outside of the reaction chamber in which the steam oxidation process is to take place. In the case of a torch, a hydrogen containing gas and an oxygen containing gas are ignited by a flame in a reaction area at atmospheric pressure and located away from and generally in a different chamber than the chamber in which wafers are placed. The flame ignition occurs at atmospheric pressure. A problem associated with pyrogenic torch methods, is that for safety reasons only certain concentration ratios of hydrogen containing gas and oxygen containing gas can be utilized. Limiting the available gas ratio unduly restricts ones ability to generate ambients with desired concentrations of H.sub.2 O/H.sub.2 or H.sub.2 O/O.sub.2. For example, in order to keep a stable flame burning, torch methods typically require H.sub.2 :O.sub.2 ratios of more that 0.5:1 and less than 1.8:1 , respectively. Bubblers are also undesirable for moisture generation in that they can be a significant source of contamination and because they cannot accurately and reliably control the amount of moisture generated.
Another problem associated with the use of pyrogenic torches and bubblers is that these methods are not easily implemented into modern rapid thermal heating apparatuses which utilize light sources for rapid temperature ramps and reaction times measured in terms of seconds as opposed to minutes and hours. Rapid thermal heaters are preferred over resistively heated furnaces because of their excellent temperature uniformity and control provides more for uniform processing and because their short reaction times reduce the thermal budget of fabricated devices.
Thus, what is desired is a method and apparatus for generating moisture in a rapid thermal heating apparatus which does not suffer from and safety issues and which can use a full spectrum of gas mixtures as well as concentration ratios.